Titan’s internal structure PS11-A025. AOGS, Singapore 12/08/2009 Dominic Fortes & Peter Grindrod University Colle

1. Titan’s internal structure PS11-A025. AOGS, Singapore 12/08/2009 Dominic Fortes & Peter Grindrod University College London

2. Results of first four Cassini radio science flybys of Titan T11: Feb 27th 2006 T22: Dec 28th 2006 T33: Jun 29th 2007 T45: July 31st 2008 Model gravity field to degree and order three Full multi-arc solution (all four flybys) New Titan spin vector included (from Radar data) Quadrupole field is hydrostatic  use Darwin-Radau approximation to obtain C/MR2 *Based on the volume of a sphere with radius 2575.0 ± 0.5 km.

3. Simple 2-layer differentiated model, water-ice shell over ‘rock’ core + low density global ocean under 100 km thick water-ice shell (r0 = 950 kg m-3, depth = 250 km) + low density global ocean under 100 km thick water-ice shell (r0 = 1200 kg m-3, depth = 250 km)

4. Simple 2-layer differentiated model, water-ice shell over ‘rock’ core MoI = 0.34 implies a core density in range 2460 – 2570 kg m-3

5. 3-layer models with partially differentiated rocky core

6. 3-layer differentiated models with metallic inner core Assuming CI chondrite ‘rock’ density and an ocean-free shell, MoI = 0.34 allows only a very small metallic core. Fe-FeS eutectic core < 450 km radius (0.5 wt. %) Fe core < 350 km radius (0.7 wt. %)

7. Consequences of a low-density core inside Titan (1) If Titan’s core density is due to a CI-chondrite mineralogy (serpentine + clays):  Core temperature must be lower than ~ 800 K to avoid dehydration (2) If Titan’s core density is due to an admixture of anhydrous rock +ice:  Core temperature is limited by ice pressure-melting curve; lower than ~ 500 K • Previous thermal models have predicted a hot interior • Core temperatures > 1300 K • Partial melting of Fe-FeS and metallic core segregation • Extensive surface geology driven by high heat flow

8. Consequences of a low-density core inside Titan • (1) • If Titan’s core is much cooler than expected • Accretion too slow for short-lived isotopes (e.g., 26Al) to provide heating • A metallic core is probably ruled out • Titan’s young surface might be attributed to processes other than volcanism • (Fall 2008 AGU abstract by Jeff Moore) • Secular cooling  contractional tectonics mass wasting and sediment build-up • (2) • Alternatively, a hot early core may have become hydrated by pervasive circulation of water along micro-fractures •  Analogous to hydrothermal alteration of CI-chondrite parent body? (cf. Ceres??)

9. Serpentinization as a source of Titan’s methane • The methane in Titan’s atmosphere may have come from: • Primordial gas accreted in clathrates • Abiotic generation during hydration of silicates by CO2-bearing waters • Biotic generation by methanogens in a subsurface ocean • These can be tested (in principle) by measuring the 12C/13C isotope ratio in the atmosphere • The observed 12C/13C ratio varies between C-bearing species, and with latitude. • The ratio in methane (= 82) is nearly solar (= 89) suggesting a primordial source. • Both biotic and abiotic processes produce isotopically very light methane. • In fact the two processes are not distinguishable by means of carbon isotopes. • However, it is possible that loss of light methane from Titan’s stratosphere is masking the input of light methane from the interior.

10. Work to do here on Earth • Experimental studies of chondrite hydration at 1 – 5 GPa • Modelling of possible hydrothermal circulation in Titan’s core • Further thermal evolutionary modelling of serpentinite and rock + ice cores • Objectives for the Titan Saturn-System Mission (TSSM) • In-situ measurement of out-gassed methane’s 12C/13C ratio • Positive confirmation of cryovolcanism • Seismometer to identify depth of major density discontinuities in Titan’s interior: • Detect subsurface ocean, roof and floor • Detect core-mantle boundary – CRITICAL